I would like to be able to recreate the following theorem style, where the main body of a theorem/proof/definition etc. is indented by a variable amount. I would also like to end a theorem proof with a filled circle rather than a square. The labels "Definition 1.1., etc." should be vertically aligned with the first line of the definition text. How do I go about achieving this?
As a starting point, I am working with the following document.
\documentclass[11pt,a4paper]{article}
\usepackage[margin=2.5cm]{geometry}
\usepackage{xcolor}
\usepackage[calcwidth]{titlesec}
\usepackage{blindtext}
\usepackage{scrextend}
\usepackage{microtype}
\usepackage[leqno]{amsmath}
\usepackage[standard,amsmath,thmmarks]{ntheorem}
\usepackage{etoolbox}
\makeatletter
\setlength \labelsep {3.15em}
\newtheoremstyle{mymargin}%
{\item[\theorem@headerfont \llap{##1 ##2}]}%
{\item[\theorem@headerfont \llap{##1 ##2}| ##3\theorem@separator\hskip\labelsep]}%
\makeatother
% my definition
\theoremstyle{mymargin}
\theorembodyfont{}
\theoremsymbol{}
\theoremprework{}
\theorempostwork{}
\theoremseparator{.}
\newtheorem{mydefinition}{Definition}
\numberwithin{mydefinition}{section}
\newtheorem{mythm}{Theorem}
\numberwithin{mythm}{section}
\begin{document}
\section{A test section}
\begin{addmargin}[10em]{0em}\begin{mydefinition}\blindtext\end{mydefinition}
\end{addmargin}
\end{document}